1. Field of the Invention
This invention relates to a gaseous discharge light emitting element, and more particularly to a light emitting element which, being of the type employing a green emission phosphor excited by ultraviolet rays irradiated by gaseous discharge, comprises an air-tight container, a gas, a phosphor and a pair of discharge electrodes sealed therein and is appropriate for use as a small lamp or use in an image display panel.
This invention is particularly concerned with a novel combination of a gas and a phosphor sealed in the air-tight container of the light emitting element which combination effects a high radiation efficiency. In the light emitting element, the discharge gap is in the range of 0.1 to 3.0mm and the pressure of the gas sealed in the container is such that the product of the pressure and the discharge gap (hereinafter referred to as "pd product") is in the range of 30 to 300 Torr.multidot.mm.
2. Description of the Prior Art
It has been known in the art to excite a phosphor with ultraviolet rays emitted by gaseous discharge to cause the phosphor to emit light. For instance, fluorescent lamps employ a phosphor which is excited by ultraviolet rays having a wavelength of 253.7nm emitted by a gaseous discharge in a mercury vapor. When the gap between a pair of discharge electrodes within a small lamp is smaller than 3mm, the pressure of the gas sealed in the lamp is required to be as high as several ten to several hundred Torr in accordance with Paschen's law. In small lamps or image display panels in which the gap between the electrodes is smaller than 3mm, therefore, the radiation of ultraviolet rays cannot be obtained effectively since the vapor pressure of mercury sealed within the lamp together with other gases such as argon is very low. In order to effectively produce ultraviolet rays by a gaseous discharge, it is necessary to heat the discharge lamp or the image display panel with a heater or the like so as to increase the vapor pressure of mercury therein. However, this requires consumption of electric power for heating and necessitates use of a large sized heating panel when the image display panel has a large size, and accordingly it is impractical to heat the discharge lamp or the like. Further, from the point of environmental pollution it is undesirable to use a large amount of mercury. Therefore, in general, this kind of discharge lamp and other similar devices usually employ a rare gas, a hydrogen gas or a nitrogen gas or an appropriate mixture of these gases because with such gases a pressure of several ten to several hundred Torr can easily be obtained at room temperatures. The ultraviolet rays emitted by the gaseous discharge in the above mentioned gas or gas mixture have radiation spectra of high intensity mostly within the so-called vacuum ultraviolet region corresponding to the wavelength of shorter than 200nm.
As a conventional phosphor which is used in image display panels employing gaseous discharge with a discharge gap of 0.1 to 3.0mm and pd product of 30 to 300 Torr.multidot.mm and emitting green light under excitation by ultraviolet rays having a wavelength of shorter than 200nm, there has been known a manganese activated zinc silicate (Zn.sub.2 SiO.sub.4 :Mn). In this phosphor, however, an improvement is desired in its color purity and life.